There exist several extraction methods to improve productivity from oil wells. However in the upstream crude oil industry, 60% to 70% of OOIP (Original Oil In Place) are typically left in the reservoir after the use of normal primary and secondary recovery techniques (Society of Petroleum Engineers. www.spe.org). The benefits of improving extraction methods are substantial. For example, there are thousands of oil wells in Texas, USA, alone, which could benefit from improving oil production output. If it were possible to recover even 50% of the heavy oil deposits, the US could supply 50% of North American demand for another 50 to 75 years (Dr. Franklin Foster, 2006).
A well for extracting fluids from geological formations is constructed by drilling a hole from the surface toward the geological formation that contains a natural resource, and that has adequate permeability to let fluids produced in the formation flow toward the well. The well's walls are lined with a cement layer and a casing that houses and supports a production tube string coaxially installed in its interior. In addition, perforations are made in the well lining in order to connect the well with the reservoir, supplying a path or trajectory inside the formation. Tubes provide an outlet for the fluids obtained from the formation.
Typically, there are numerous perforations that extend radially from the lined or coated well. Perforations are uniformly separated in the lining, and pass to the outside of the lining through the formation. In an ideal case, perforations are only located within the formation, and their number depends on the formation thickness. It is rather common to have nine, and up to twelve perforations per depth meter of formation. Other perforations extend longitudinally, and yet other perforations may extend radially from a 0°-azimuth, while additional perforations, located every 90° may define four sets of perforations around azimuth. Formation fluids pass through these perforations and come into the lined (or coated) well.
Preferably, the oil well is plugged by a sealing mechanism, such as a shutter element, or with a bridge-type plug, located below the level of perforations. This shutter element is connected to a production tube, and defines a compartment. The production fluid, coming from the formation or reservoir, enters the compartment and fills the compartment until it reaches a fluid level. Accumulated oil, for example, flows from the formation and can be accompanied by variable quantities of natural gas. Hence, the lined compartment may contain oil, some water, natural gas, and solid particles, with normally, particles settling at the bottom of the compartment.
The fluid produced in the formation may change its phase when there is a reduction of pressure around the well; this change of phase causes the gasification of the lightest molecules. Also, the oil well can produce very heavy molecules. Over time, due to several reasons, oil well productivity gradually diminishes. Two main causes of the reduction in productivity are related to relative permeability: a decrease of the fluidity of crude oil, and the deposit of solids in the perforations.
Crude oil's fluidity diminishes over time and progressively obstructs pores in a deposit or reservoir. On the other hand, solids such as clays, colloids, salts, paraffin etc. accumulate in perforation zones of the well. These solids reduce the absolute permeability, or interconnection between pores. Problems associated with the causes mentioned above are: obstruction of pores by mineral particles that flow jointly with the fluid to be extracted, precipitation of inorganic scales, decanting of paraffins and asphalt or bitumen, hydration of clay, invasion of solids from the mud and filtration of perforation mud, as well as invasion of termination fluids and solids from brine injections. Each of the above mentioned causes can produce a permeability reduction, or a flow restriction in the zone surrounding oil well perforations. This defines the pore size connecting to the fluid inside formation, allowing the fluid flow from the formation through cracks or fissures, or connected pores, and finally the fluid comes to interstitial spaces within the compartment and is collected. During that flow, very small solid particles from the formation, called “fines,” may flow; but instead they tend to settle.
After a certain time, trajectories through perforations extending inside the formation of a reservoir may become obstructed with “fines” or residues. While the “fines” can be kept in a disperse state for some time, they can agglomerate and plug the pore space, reducing the fluid rate or production quantity. This may become a problem that is fed back to the well and cause a production decrease. More and more “fines” can keep settling on perforations, plugging them more and more, even tending to halt a minimum flow rate.
There exist several treatment methods to improve productivity from oil wells. Periodic stimulation of oil and gas wells is done by applying three general types of treatment: acid treatment, fracturing, and default treatment with solvents and heat. Acid treatment consists of using mixtures of acids HCl and HF (hydrochloric acid and hydrofluoric acid), which is injected in the production zone (rock). Acid is used for dissolving reactive components (carbonates, clay minerals, and in a smaller quantity, silicates) in the rock, thus increasing permeability. Frequently, additives are incorporated, such as reaction retarding agents and solvents, to improve acid performance in the acidizing operation.
While acid treatment is a common treatment to stimulate oil and gas wells, this treatment has multiple drawbacks. The cost of acids and the cost of disposing of production wastes are high; acids are often incompatible with the crude oil, and may produce viscous oily residues inside the well; precipitates formed once the acid is consumed, can often be more obnoxious than dissolved minerals; and the penetration depth of active or live acid is generally less than 5 inches (12.7 cm).
Hydraulic fracturing is another technique usually used for stimulating oil and gas wells. In this process, high hydraulic pressures are used to produce vertical fractures in the formation. Fractures can be filled with polymer plugs, or treated with acid (in rocks, carbonates, and soft rocks), to form permeability channels inside the wellbore region; these channels allow oil and gas to flow. However, the cost of hydraulic fracturing is extremely high (as much as 5 to 10 times higher than acid treatment costs). In some cases, fracture may extend inside areas where water is present, thus increasing the quantity of water produced (a significant drawback for oil extraction). Hydraulic fracture treatments extend several hundred meters from the well, and are used more frequently when rocks are of low permeability. The possibility of forming successful polymer plugs in all fractures is usually limited, and problems such as plugging of fractures and grinding of the plug may severely deteriorate productivity of hydraulic fractures.
Another method for improving oil production in wells involves injecting steam. One of the most common problems in depleted oil wells is precipitation of paraffin and asphaltenes or bitumen inside and around the well. Steam has been injected in these wells to melt and dissolve paraffin into the oil or petroleum, and then all the mixture flows to the surface. Frequently, organic solvents are used (such as xylene) to remove asphaltenes or bitumen whose melting point is high, and which are insoluble in alkanes. Steam and solvents are very costly (solvents more so than steam), particularly when marginal wells are treated, producing less than 10 oil barrels per day (1 bbl=159 liters). The main limitation for use of steam and solvents is the absence of mechanical mixing, which is required for dissolving or maintaining paraffin, asphaltenes or bitumen in suspension.
Several other methods have been described to increase oil well output. Challacombe (U.S. Pat. No. 3,721,297) describes a tool for cleaning wells using pressure pulses: a series of explosive and gas generator modules are interconnected in a chain, in such a manner that ignition of one of them triggers the next one and a progression or sequence of explosions is produced. These explosions generate shock waves that clean the wells. There are obvious disadvantages of this method, such as potential damage that can be caused to high-pressure oil and gas wells.
Sawyer (U.S. Pat. No. 3,648,769) describes a hydraulically controlled diaphragm that produces “sinusoidal vibrations in the low acoustic range”. Generated waves are of low intensity, and are not directed or focused to face the formation (rock). As a consequence, the major part of energy is propagated along the well axis.
Riggs et al. (U.S. Pat. No. 4,343,356) describes an apparatus for treating shallow wells. Application of a high voltage produces voltage arcs that liberate from the well walls the encrusted material. Among difficulties with this apparatus there is the fact that it is not possible to continually guide the electric arc for achieving a real cleaning of the well. Additionally, safety aspects have not been solved (electrical and fire problems).
Bodine (U.S. Pat. No. 4,280,557) proposes another hydraulic/mechanical oscillator where pulses of hydraulic pressure, created, inside an elastic elongated tube, are used for cleaning encased well walls.
Mac Manus et al. (U.S. Pat. No. 4,538,682) disclosed a method for removing paraffin from oil wells by introducing a heating element into an oil well in order to establish a temperature gradient within the well.
It is known that oil, gas, and water wells are plugged after certain operating time; the fluid discharge diminishes and it becomes necessary to regenerate these wells. Mechanical, chemical, and conventional techniques to regenerate wells include: intensive rinsing, pumping hammer and hydraulic treatment.
Dissolution of sediments using hydrochloric acid, or other acids, mixed with other chemical agents include: Hosing down with high-pressure water, carbon dioxide injection, and generation of pressure shocks using explosives.
Ultrasound techniques have been developed to increase production of crude oil from wells. However, there is a great amount of effects associated with exposing solids and fluids to an ultrasound field of certain frequencies and energy. In the case of fluids in particular, cavitation bubbles can be generated. These are bubbles of gas dissolved in liquid, or bubbles of the gaseous state of this liquid (change of phase). Other associated phenomena are liquid degassing and cleaning of solid surfaces.
Arthur Kuris, in “Method and Apparatus for Fracturing Rock and the Like” (U.S. Pat. No. 3,990,512), discloses a method for recovering oil by application of ultrasound generated when injecting high-pressure fluids, whose purpose is to fracture the deposit to produce new draining channels.
Maki Jr. et al. (U.S. Pat. No. 5,595,243) propose an acoustic device in which a piezoceramic transducer is set as radiator. The device presents difficulties in its manufacturing and use, because an asynchronous operation is required of a high number of piezoceramic radiators.
Vladimir Abramov et al., in “Device for Transferring Ultrasonic Energy to a Liquid or Pasty Medium” (U.S. Pat. No. 5,994,818) and in “Device for Transmitting Ultrasonic Energy to a Liquid or Pasty Medium” (U.S. Pat. No. 6,429,575), propose an apparatus consisting of an alternating current generator operating within the range of 1 to 100 kHz to transmit ultrasonic energy, and a piezoceramic or magnetostrictive transducer emitting ultrasound waves, which are transformed by a tubular resonator or waveguide system (or sonotrode) in transversal oscillations that contact the irradiated liquid or pasty medium. However, these systems are conceived to be used in containers of very large dimensions, at least as compared with the size and geometry of perforations present in wells. This shows limitations from a dimensional point of view, and also for transmission mode if it is desired to enhance production capacities of oil wells.
Julie C. Slaughter et al., in “Ultrasound Radiator of Downhole Type and Method for Using It” (In U.S. Pat. No. 6,230,788), propose a device that uses ultrasonic transducers manufactured of Terfenol-D alloy and placed at the well bottom, and fed by an ultrasonic generator located at the surface. Location of transducers, axially to the device, allows the emission along a transversal direction. This invention proposes a viscosity reduction of hydrocarbons contained in the well through emulsification, when reacting with an alkaline solution injected to the well. This device considers a forced shallow circulation of fluid as a refrigeration system, to warrant continuity of irradiation.
Dennos C. Wegener et al., in “Heavy Oil Viscosity Reduction and Production,” (U.S. Pat. No. 6,279,653), describe a method and a device for producing heavy oil (API specific gravity less than 20) applying ultrasound generated by a transducer made of Terfenol alloy, attached to a conventional extraction pump, and powered by a generator installed at the surface. In this invention the presence of an alkaline solution is also considered, similar to an aqueous sodium hydroxide (NaOH) solution, to generate an emulsion with crude oil of lower density and viscosity, thereby facilitating recovery of the crude by impulsion with a pump. Here, a transducer is installed in an axial position to produce longitudinal ultrasound emissions. The transducer is connected to an adjacent rod that operates as a waveguide or sonotrode.
Robert J. Meyer et al. , in “Method for improving Oil Recovery Using an Ultrasonic Technique” (U.S. Pat. No 6,405,796), propose a method to recover oil using an ultrasound technique. The proposed method consists of disintegration of agglomerates by means of an ultrasonic irradiation technique, and the operation is proposed within a certain frequency range, for the purpose of handling fluids and solids in different conditions. Main oil recovery mechanism is based in the relative momentum of these components within the device.
The above-mentioned prior art using ultrasonic waves via a transducer, externally supplied by an electric generator and the transmission cable generally is longer than 2 km. This has the disadvantage of signal transmission losses, which means that a signal must be generated to have enough intensity (or energy) for an adequate operation of transducers within the well, since high-frequency electric current transmission to such depths is reduced to 10% of its initial value. Furthermore, since transducers need to operate at a high-power regime, water or air cooling system is required, which poses great difficulties when placed inside the well. The latter implies that ultrasound intensity must not exceed 0.5-0.6 W/cm2. This level is insufficient for the desired purposes, because threshold of acoustic effects in oil and rocks is from 0.8 to 1 W/cm2.
Andrey a. Pechkov, in “Method for Acoustic Stimulation of Wellbore Bottom Zone for Production Formation” (RU Patent No. 2,026,969), disclose methods and devices for stimulating production of fluids within a producing well. These devices incorporate, as an innovating element, an electric generator attached to the transducer, and both of them integrated in the well bottom. These transducers operate in a non-continuous mode, and can operate without needing an external cooling system. The impossibility of operating in a continuous mode to prevent overheating is one of the main drawbacks of this implementation since the availability of the device is reduced. Moreover, as the generator is located in the well bottom, this equipment maintenance cost rises as it is likely to fail, especially when working in high power applications.
Oleg Abramov et al., in “Acoustic Method for Recovery of Wells, and Apparatus for its Implementation” (U.S. Pat. No. 7,063,144), disclose an electro-acoustic method for stimulation of production within an oil well. The method consists of stimulating, by powerful ultrasound waves, the well extraction zone, causing an increase of mass transfer through its walls. This ultrasonic field produces large tension and pressure waves in the formation, thus facilitating the passage of liquids through well orifices. It also prevents accumulation of “fines” on these holes, thereby increasing the life of the well and its extraction capacity.
Some problems encountered in the solutions proposed by Robert J. Meyer et al., in “Method for improving Oil Recovery Using an Ultrasonic Technique” (U.S. Pat. No 6,405,796), Andrey A. Pechkov, in “Method for Acoustic Stimulation of Wellbore Bottom Zone for Production Formation” (RU Patent No. 2,026,969), Dennos C. Wegener et al., in “Heavy Oil Viscosity Reduction and Production,” (U.S. Pat. No. 6,279,653), Oleg Abramov et al., in “Acoustic Method for Recovery of Wells, and Apparatus for its Implementation” (U.S. Pat. No. 7,063,144) and Julie C. Slaughter et al., in “Ultrasound Radiator of Dowhole Type and Method for Using It” (In U.S. Pat. No. 6,230,788), are:
a) some devices to be introduced in the well containing the ultrasound radiator are sensible to degradation by hydrocarbons and corrosion by acids present at the well bottom;
b) some devices are not intended to be used in oil/water wells with high content or presence of gas, due to their almost null capacity to dissipate the heat generated by the mechanic wave radiators when said radiators are not in contact with liquid fluids, situation that eventually will destroy the radiators or other components (cables, wires, coils, others); and
c) some devices are not meant to be used in Gas Reservoirs or Gas wells.
d) some devices have associated environmental treatment costs due to the use of chemicals.
Therefore, what is needed is a method, apparatus and system for improving well productivity that does not present (or at least minimizes) the drawbacks of the existing technologies. The invention provides a system, apparatus and method for increasing production capacity of oil, gas and water wells.